Abstract Detail

The Effect of Environment and Ecology on the Deposition of Silica in Grasses.

The uptake and deposition of silica in aboveground tissues is common among land plants. The degree of deposition, however, varies considerably between groups and is controlled by a combination of genetic and environmental factors. A number of groups, including grasses, characteristically accumulate large amounts of silica. High silica concentrations are associated with a variety of benefits, ranging from the alleviation of drought stress and heavy metal toxicity, to herbivore defense. The latter has received extensive study in grasses, and has been incorporated into a number of macroevolutionary hypotheses about the group. Among these is the hypothesis that C4 lineages have evolved to accumulate higher concentrations of silica than C3 grasses, in response to a history of extensive predation from herbivores (particularly large mammals) in the former. This hypothesis is based partly on the observation that large, mammalian herbivores are, today, most abundant in habitats dominated by grasses possessing the C4 photosynthetic pathway. We attempted to test this using a large comparative dataset compiled from grasses grown under controlled environmental conditions. Leaf material was collected, from grasses grown in growth chambers under three sets of conditions, and was subjected to elemental analysis using a portable X-ray fluorescence spectrometer. Silicon concentration (proxy for silica) estimates were recorded, and analyzed within a phylogenetic framework. Results consistently rejected the hypothesis that C4 taxa accumulate more silica, regardless of environmental conditions. A more general hypothesis that tested the influence of habitat type (open vs. closed) was also rejected. Reductions in temperature and water availability were associated with an average, 35% reduction and 22% increase in concentrations, respectively, with results largely consistent across taxonomic and ecological groups. In aggregate, our results suggest that silica defenses did not evolve differently in C3 and C4 grasses, nor are they fundamentally different between grasses of open and closed habitats. This may be the result of similar exposure to herbivores that are deterred by silica, and/or because changes in silica concentration, over evolutionary timescales, are controlled by factors other than defense. The tendency for C4 grasses to be more abundant under conditions, found here to promote high silica concentrations, may explain previous observations on silica levels in each group. Because silica deposition responds to changes in temperature and water availability, spatiotemporal variation in climate may be an important impact on the deposition of silica, and may also complicate comparative work on macroevolutionary trends in silica deposition.